Ooh, it just makes me mad when people assume. One time a couple of years ago (I think my dd was about 1.5 years old), we were out to dinner with some of dh's relatives and one of them, whom I'd only just met, asked me if I was expecting. Um, no. The conversation kind of died after that for a bit...
Then about a year ago (dd was 2.5 yo) a lady at church asked me when I was due. This wasn't a lady that I saw a lot, but we knew each other, and she and her dh are good friends with my IL's. I said, "Sorry?" In case I misunderstood her. She asked again when I was due, and I said, "for what?" And she looked really embarassed and said, "Oh, you're not expecting? I thought someone told me..." blah blah blah and made up a few excuses as she walked away. It was really embarassing for both of us. Really hurts the old self esteem, too. Sure, at that time I WANTED to be pregnant, but I didn't want to look it if I wasn't, KWIM?